The present invention relates to structured catalysts, and more particularly to structured catalysts of honeycomb or other open cross-flow shape wherein the catalysts are supported on the interior surfaces of the structures by catalyst support or washcoating layers configured to improve catalyst effectiveness for the treatment of fluid reactant streams passing through the structure.
Inorganic honeycombs supporting catalyst coatings on the interior channel walls of the honeycombs are widely used for applications such as automobile engine exhaust emissions control. In general, the catalysts for these applications are manufactured by washcoating selected ceramic or metallic honeycomb structures with slurries comprising refractory, high-surface-area catalyst support oxides, and then depositing selected metallic catalysts on the surfaces of the support oxides. Thin washcoats of alumina supporting precious metal catalysts such as platinum, palladium and rhodium these applications are disclosed, for example, in U.S. Pat. Nos. 4,762,567 and 4,429,718.
A related coating technology, more commonly used to deposit thin and usually dense oxide coatings on substrates, is sol-gel processing. U.S. Pat. No. 5,210,062 discloses the use of oxide sols to deposit thin washcoats on honeycomb supports for automotive catalytic converters. Sol-gel coatings have also been used, for example, to protect underlying substrate materials, to improve wear resistance, and to impart desired dielectric properties. The starting materials for these processes are liquid sols, which may be defined as liquid suspensions of solid particles that are small enough to remain suspended indefinitely by Brownian motion. In the sol-gel process, these sols are converted to gels by appropriate chemical or thermal treatments during which solid or semi-solid networks of the solid particles are formed, with the liquid phases being uniformly interspersed throughout.
Gels produced in this way can exhibit viscous flow behavior permitting shaping into useable forms such as bulk shapes, fibers, coatings and the like. Oxide films can be prepared from the gels or their precursor sols by methods such as spin, dip, spray, bead, slot, curtain or brush coating, with subsequent heating to remove the liquids and convert the solids to thin and/or dense oxide coatings of a variety of predetermined compositions and structures.
One common sol-gel approach for producing protective oxide coatings, disclosed for example in U.S. Pat. Nos. 4,921,731 and 5,585,136 and in published PCT application No. WO 01/16052, is to dissolve organometallic precursor compounds of the desired oxides in a suitable solvent and to hydrolyze the organometallics to form the sol. This sol is subsequently converted by chemical treatment or heating to an organic-inorganic gel comprising the solvent, oxide particles, and organo-metallic polymers or clusters. Coatings of these gels can then be further heated to convert them to oxide coatings. The tendency of such sol-gel oxide coatings to crack during heating is reduced through the use of particulate oxide fillers of appropriate composition. Where increased coating density is required, a treatment using phosphate components is used.
The use of sols as oxide powder binders for thin oxide catalyst support coatings has also been proposed. U.S. Pat. No. 3,928,239, for example, employs a permanent binder of an inorganic acid alumina sol in an alumina washcoat for an automotive or stationary exhaust gas oxidation catalyst. Further, published PCT application No. WO 95/23025, discloses sol-based oxide underlayers for bonding conventional metal-oxide catalyst coatings to metallic catalyst supports.
An important goal of much of the washcoating technology developed for the support of precious metal automobile and other exhaust emissions control catalysts has been the protection of the oxidation activity of the largely surface-concentrated catalyst deposits at high operating temperatures and high gas flow rates. Coating thicknesses are minimized to reduce system back-pressures; thicker coatings confer no advantage and are avoided for this reason. Thus adherent washcoats of relatively high surface area and refractoriness, but relatively low thickness and porosity, have been used. Unfortunately, such catalysts and washcoats are not very effective for promoting other types of reactions, including many reactions requiring longer reactant diffusion paths or involving mixed gas/liquid reaction streams.